Chromium-base alloy



Nov. 14, 1961 N. J. GRANT ETAL CHROMIUM-BASE ALLOY Filed Oct. 21, '1958 FIG.|

400 TRANSITION EMPERATURE F M! w M 0 R "I c D E H R U I 2 H w O O czum m0 M4024 ATOMIC /u ELEMENT Nicholas J. Grant Ernest P. Abrahomson ]I INVENTORS ATOMIC '7. ALLOY ELEMENT AGENT United States Patent M 3,008,854 CHROMIUM-BASE ALLOY Nicholas J. Grant, Leslie Road, Winchester, Mass, and Ernest P. Abrahamson 11, Watertown, Mass.; said Abrahamson assignor to said Grant Filed Oct. 21, 1958, Ser. No. 768,714 21 Claims. (Cl. 14832) This invention relates to chromium-base alloys characterized by improved temperature range of ductility and in particular to a method for depressing the ductile to brittle transition temperature of chromium metal and chromium alloys.

Chromium-base alloys are attractive high temperature materials from an oxidation standpoint. However, a major disadvantage is their lack of ductility and malleability at room and at certain elevated temperatures.

Depending on the amount of impurity present, various varieties of chromium may have different ductile to brittle transition temperatures. For example, one variety of chromium may have a transition temperature in the neighborhood of about 500 F. while another variety might exhibit a transition temperature in the neighborhood of about 1000 F. On work conducted heretofore, the addition of a second element to chromium was found to raise the ductile to brittle transition temperature which added to rather than alleviated metallurgical processing difficulties. For example, nitrogen in small amounts, in the absence of other elements, 'was found to increase markedly the transition temperature.

It is desirable to add other alloying elements to chromium in order to improve its physical properties at room and elevated temperatures. However, additions of such elements as tungsten, molybdenum, nickel, iron, etc. render the chromium alloy so unworkable that it must be cast directly into the shape desired.

Because chromium and its alloys show great promise as a high temperature material, it would be desirable to control'the ductile to brittle transition temperature, that is to depress it, whereby to increase the temperature range of malleability and ductility and thus improve the metallurgical processing of the material while at the same time improve certain of the physical properties, such as impact resistance (thermal and mechanical) and resistance to fatigue, tension, etc.

' An object, therefore, is to provide a chromium-base material characterized by an improved temperature range of ductility.

Another object is to provide a method for depressing the ductile to brittle transition temperature of chromium in order to improve its workability as well as improve the workability of the alloys produced therefrom.

These and other objects will more clearly appear from the following disclosure when taken together with the drawings wherein:

FIG. 1 depicts a plot based on typical bend test data showing the effect of small amounts of nitrogen on the brittle to ductile transition temperature of chromium;

FIG. 2 illustrates the effect of other additives in amounts up to about 6 atomic percent on the transition temperature, including certain additives contemplated by the invention; and

FIG. 3 is similar to FIG. 2 in that it shows the effect of certain additives on the transition temperature of chromium including those additives contemplated by the invention, in amounts ranging up to about 30 atomic percent.

We have found that the ductility limitations inherent in chromium metal and chromium-base alloys can be alleviated by adding to the chromium a metal from the 3,008,854 Patented Nov. 14, 1961 group ruthenium, palladium, iridium and platinum in an amount at least suflicient to depress the ductile to brittle transition temperature of said chromium. The expression transition temperature as used herein is defined as that temperature at about which the ductile to brittle transition occurs. Such transition may be determined by a bend test, a tensile test, a compressive test or a torsion test or the like. We have found a bend test to be suitable for our purposes.

As illustrative of the foregoing, reference is made to FIG. 1 which is a plot showing the effect of nitrogen on the ductile to brittle transition temperature of hydrogen purified electrolytic chromium. The ductile to brittle transition temperature was measured by means of a bend test.

Two grades of electrolytic chromium were tested: one containing 0.008 weight percent nitrogen and the other containing 0.015%. Each was melted under substantially non-reactive conditions, e.g. under purified argon in a water-cooled copper crucible, tungsten electrode arc furnace. After the desired molten bath was obtained, the metal was cast into bend test rods by sucking up the molten metal into a quartz tubing having a one-eighth inch inside diameter. This method of bottom casting assured a relatively fine and constant grain size for all castings. The rods were then electro-polished to insure removal of any outer layer of silicon contamination arising from the casting which tends to raise the transition temperature by up to several hundred F. All rods were tested in the as cast condition since this was the most unfavorable condition sensitivitywise from the standpoint of transition temperature.

The rods were then subjected to bend tests at various temperatures using a test apparatus consisting of a double specimen support giving a span of about one inch and a load rod, both enclosed in a furnace capable of obtaining 2200 F. The testing temperature was measured at therod specimen. All specimens 'were held at the testing temperature for about five minutes prior to bending. Under these conditions nitrogen pickup was not indicated which was confirmed by vacuum fusion analysis.

The load was applied at a strain rate of about 0.1 to 0.01 inch per minute. At each temperature, the specimen was bent to fracture and the angle of bend after fracture measured. The bend test results were then plotted as the angle of bend versus the test temperature.

Referring to FIG. 1, it will be noted that raising the nitrogen content from 0.008% by weight to 0.015% raises the transition temperature by about 700 F. The slopes of both curves are substantially equal. The maximum bend of 130 was obtained for the 0.008% nitrogen chromium starting at about 400 F. while the maximum angle of bend was obtained for the 0.015% nitrogen chromium starting at about 0" F.

It will be noted that the values of the transition temperature approximate a straight line. As a standard for future testing, a 65-degree bend was chosen as the arbitrary transition temperature. Thus the temperature at which this amount of ductility was observed was plotted against the atomic percent of the alloying solute element as shown in FIGS. 2 and 3.

-The effect of alloying solute elements was determined on several grades of hydrogen purified electrolytic chromium which contained by weight from 0.012 to0.02% oxygen, 0.008 to 0.015% nitrogen, 0.006 to 0.010% carbon,

and 0.0004 to 0.0006% hydrogen.

The alloys were produced by using the same melting and casting procedures described hereinbefore. The analyses of the solute elements employed in producing the various alloy compositions are given in the following table:

TABLE I Composition of elements used for alloying with chromium Min. Percent Weight Percent Impurity After electropolishing the cast alloy rods, each was subjected to a bend test to determine the temperature at which the 65 degree bend occurred.

Referring to FIG. 2, it will be noted that ruthenium has an immediate beneficial effect in depressing the duetile to brittle transition temperature, this temperature being lowered from about 350 F. to about 100 F. when an amount of ruthenium in the neighborhood of six atomic percent was added. All other elements effected an immediate increase in the transition temperatures when added to the chromium. However, with respect to solute elements Pd, Ir and Pt it will be noted that at about 3 atomic percent, the rate of increase in the transition temperature falls off markedly whereby above 3 the transition temperature drops rapidly to low values with increased amounts o-f solute elements up to 30 atomic percent.

The foregoing will appear more clearly by referring to FIG. 3 which illustrates the effect of addition of solute elements in amounts up to 30 atomic percent with respect to Pd, Ir and Pt, a marked reduction in transition tem perature is effected between 3 and 30 atomic percent of the element. It is apparent from FIG. 3, therefore, that the elements Ru, Pd, Ir and Pt bring about the greatest decrease in transition temperature for chromium metal.

In deriving the data illustrated graphically in FIG. 3, two grades of chromium were employed, each having a diiferent transition temperature. The two chromiumpalladium alloys shown in FIG. 3 (Pd-1 and Pd-2) indicate that the transition temperature of a binary alloy is dependent on the transition temperature of the original chromium as well as the particular element added. It will be noted that both curves reach a maximum in transition temperature at approximately 3 atomic percent palladium, the slope of the curve being maintained from one grade of chromium to another, the slope of each being transposed by an amount equal to the difference in transition temperature of the two chromuim grades. This brings out the importance of further purifying the chromium to lower the transition temperature as much as possible so as to take advantage of the further temperature depressing propensity of the alloying element at minimum cost and to provide for judicious alloying to achieve other important physical and mechanical properties, such as creep resistance, tensile strength, etc.

The transition temperature curve for irridiurn likewise reaches a maximum in the neighborhood of about 3 atomic percent as shown by the dotted line running parallel to the ordinate and which passes through each of the maxima of the iridium curve and the Pd-l and Pd-2 curves. Thus, it is apparent from FIG. 3 that the ductility of chromium can be greatly improved by adding an alloying element selected from the group consisting of Ru, Pd, Ir and Pt, in an amount at least sufiicient to depress the ductile to brittle transition temperature, said amount ranging up to about 30 atomic percent, preferably above 3% to about 30%. A more preferred range would be in the neighborhood of about 4 to 20%, depending upon the purity and transition temperature of the chromium itself.

With respect to ruthenium which has an immediate de- 5 pressing effect on the transition temperature, it may be added in amounts ranging from about 0.5% to about 30 atomic percent. Iridium may be employed in amounts over 3 and up to about atomic percent, preferably 7 to 20 atomic percent. Palladium likewise may range from over 3 up to about 20 atomic percent and preferably from about 7 to 20 atomic percent.

The invention is applicable to chromium metal containing 95 or 98% by weight of chromium and higher. Generally speaking, the invention is applicable to electrolytic chromium of substantially high purity, for example chromium consisting of at least about 99.9 weight percent. To achieve the maximum benefit of the depressing effect of the alloying element, the nitrogen content of the electrolytic chromium should be as low as is feasi- 20 bly possible. For example, the nitrogen content should be maintained below 0.01 weight percent, and preferably not exceed 0.005% and, if possible, not exceed 0.001%. The same is true of chromium-base alloys produced from electrolytic chromium.

The invention is also applicable to chromium-rich alloys of poor ductility, for example, alloys containing at least 70 weight percent of chromium, such as an alloy containing 70% Cr, 12% Ni, 15% Fe and 3% Ta.

As in other alloying systems, the addition of alloying ingredients (e.g. metals of group IVa, Va, VIa, VIIa and the iron group of the periodic system of elements) to chromium is important where improved tensile strength, hardness, resistance to creep, etc. is desired to meet certain requirements in the field. Unfortunately, additions of such alloying ingredients as cobalt, iron, nickel, tungsten, molybdenum and the like, forming binary, ternary, quaternary and other complex alloy systems, generally raise the ductile to brittle transition temperature: of the resultant chromium-base alloy to values incompatible with conventional working temperatures whereby such alloys could only be used in the cast condition.

Referring to FIG. 3, it will be noted that the addition of several atomic percent of nickel raised the transition temperature to a maximum of about 2250 F. which thus rendered the forging or even the cold working of the alloy very difiicult. Additions of cobalt raised the ductile to brittle transition temperature to about 1700 C. while several percent of iron raised it to 1500 F. However, where the presence of these and other elements are de sired for other reasons (improved strength, hardness, etc.), the lack of malleabi-lity can be compensated for by utilizing the teaching of the present invention. As illustrative of the foregoing, the following table of data is given:

Atomic Percent Element Transi- Test No. tion Temp,

Cr 00 Fe Ni Pd Ru F.

Tests Nos. 1 and 2 show the effect of 5.1 and 5.4 atomic percent palladium, respectively, in depressing the transition temperature of binary alloys of Cr-Fe to as low as 870 F. and 820 F.

The addition of 14 atomic percent palladium to a chromium-nickel binary containing 5.2 and 15.8 atomic percent nickel (Nos. 3 and 4) depressed the transition temperature down to 280 and 300 F., respectively.

Simi ar results are indicated for the depressing effect of ruthenium on Cr-Co- (No. 5). 7 It is apparent from the foregoing that the invention is applicable to chromium-base alloys as well as to chromium metal itself. By improving the ductility of the chromium, or alloys based on chromium, the metal is rendered more amenable to metallurgical processing, for example, in metallurgical and mechanical processes involving the shaping of metal into turbine blades and the like by drop forging and other working operations.

While the present invention has been described as being applicable to chromium metal and to certain chromium binary alloys, results have also indicated the invention to be applicable to complex chromium alloys of limited malleability in which chromium is the predominating element. Thus, where chromium is defined in the claims as making up substantially or essentially the balance of the alloy, it is meant to be the chief ingredient, for example at least about 70% of the total alloy composition. The foregoing does not exclude the presence of such alloying elements from groups IVa, Va, VIa, WM and the iron group metals and other elements.

Although the present invention has bee-n described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A method for improving the malleability of a chromium-base alloy by depressing the ductile to brittle transition temperature of said alloy whereby to increase its temperature range of malleability which comprises, establishing a chromium-containing molten bath under substantially non-reactive conditions, alloying with said bath at least one metal selected from the group consisting of Ru, Pd, Ir and Pt in an amount ranging up to about 30 atomic percent, said amount being at least sufiicient to depress the ductile to brittle transition temperature of the resulting composition, said alloy containing at least 70% chromium, casting the resulting alloy, and working said alloy at above said depressed transition temperature.

2. A method for improving the malleability of a chromium-base alloy by depressing the ductile to brittle transition temperature of said alloy whereby to increase its temperature range of malleability which comprises establishing a chromium-containing molten metal bath under substantially non-reactive conditions, alloying with said bath at least one metal selected from the group consisting of Ru, Pd, Ir and Pt in an amount ranging from over 3 to about 30 atomic percent, said 'alloy containing at least 70% chromium, casting the resulting alloy, and working said alloy at above said depressed transition temperature.

3. A method for improving the malleability or a chromium-base alloy by depressing the ductile to brittle transition temperature of said alloy whereby to increase its temperature range of malleability which comprises, establishing a chromium-containing molten metal bath under substantially non-reactive conditions, alloying with said bath at least one metal selected from the group consisting of Ru, Pd, Ir and Pt in an amount ranging from about 4 to 20 atomic percent, said alloy containing at least 70% chromium, casting the resulting alloy, and working said alloy at above said depressed transition temperature.

4. A method for improving the malleability of chromium metal constituted of at least 95% by weight of chromium by depressing the ductile to brittle transition temperature of said chromium whereby to increase its temperature range of malleability which comprises, establishing a molten bath of said chromium metal under substantially non-reactive conditions, alloying with said bath at least one metal selected from the group consisting of Ru, Pd, Ir and Pt in an amount ranging up to about 30 atomic percent of the final alloy composition, said amount being at least sufiicient to depress the brittle to ductile transition temperature of said chromium, casting the resulting alloy, and working said alloy at above said depressed transition temperature.

5. A method for improving the malleability of chromium metal constituted of at least by Weight of chromium by depressing the ductile to brittle transition temperature of said chromium whereby to increase its temperature range of malleability which comprises, establishing a molten bath of said chromium metal under substantially non-reactive conditions, alloying with said bath at least one metal selected from the group consisting of Ru, Pd, Ir and Pt in an amount ranging from over 3 to about 30 atomic percent of the final alloy composition, casting the resulting alloy, and working said alloy at above said depressed transition temperature.

6. A method for improving the malleability of chromium metal constituted of at least 95% by weight of chromium by depressing the ductile to brittle transition temperature of said chromium whereby to increase its temperature range of malleability which comprises, establishing a molten bath of said chromium metal under substantially non-reactive conditions, alloying with said bath at least one metal selected from the group consisting of Ru, Pd, Ir and Pt in an amount ranging from about 4 to 20 atomic percent of the final alloy composition, casting the resulting alloy, and working said alloy at above said depressed transition temperature.

7. A method for improving the malleability of chromium metal constituted of at least 95% by weight of chromium by depressing the ductile to brittle transition temperature of said chromium whereby to increase its temperature range of malleability which comprises, esta'blishing a molten bath of said chromium metal under substantially non-reactive conditions, alloying ruthenium with said bath in an amount ranging from about 0.5 to 30 atomic percent of the final alloy composition to depress the ductile to brittle transition temperature of said chromium, casting the resulting alloy, and working said alloy at above said depressed transition temperature.

8. A method for improving the malleability of chromium metal constituted of at least 95% by weight of chromium by depressing the ductile to brittle transition temperature of said chromium whereby to increase its temperature range of malleability which comprises, establishing a molten bath of said chromium metal under substantially non-reactive conditions, alloying ruthenium with said bath in an amount ranging from about 1 to 10 atomic percent of the final alloy composition to depress the ductile to brittle transition temperature of said chromium, casting the resulting alloy, and working said alloy at above said depressed transition temperature.

9. A method for improving the malleability of chromium metal constituted of at least 95% by Weight of chromium by depressing the ductile to brittle transition temperature of said chromium whereby to increase its temperature range of malleability, which comprises establishing a molten bath of said chromium metal under substantially non-reactive conditions, alloying iridium with said bath in an amount ranging from over about 3 to about 20 atomic percent of the final alloy composition, casting the resulting alloy, and working said alloy at above said depressed transition temperature.

10. A method for improving the malleability of chromium metal constituted of at least 95% by weight of chromium by depressing the ductile to brittle transition temperature of said chromium whereby to increase its temperature range of malleability which comprises establishing a molten bath of said chromium metal under substantially non-reactive conditions, alloying iridium with said bath in an amount ranging from about 7 to 20 atomic percent of the final alloy composition to depress the ductile to brittle transition temperature of said chromium, casting the resulting alloy, and working said alloy at above said depressed transition temperature.

11. A method for improving the malleability of chromium metal constituted of at least 95% by weight of chromium by depressing the ductile to brittle transition aooesea temperature of said chromium whereby to increase its temperature range of malleability which comprises, establishing a molten bath of said chromium metal under substantially non-reactive conditions, alloying palladium with said bath in an amount ranging from over about 3 to about 20 atomic percent of the final alloy composition, casting the resulting alloy, and working said alloy at above said depressed transition temperature.

12. A method for improving the malleability of chromium metal constituted of at least 95% by weight of chromium by depressing the ductile to brittle transition temperature of said chromium whereby to increase its temperature range of malleability which comprises, establishing a molten bath of said chromium metal under substantially non-reactive conditions, alioying palladium with said bath in an amount ranging from about 7 to 20 atomic percent of the final alloy composition, casting the resulting alloy, and working said alloy at above said depressed transition temperature.

13. A wrought chromium-base alloy characterized by a depressed ductile to brittle transition temperature whereby it has an improved temperature range of malleability, said alloy containing an amount of at least one metal selected from the group consisting of Ru, Pd, Ir and Pt ranging up to about 30 atomic percent at least suflicient to depress the ductile to brittle transition temperature of said alloy, less than 0.01% by weight of nitrogen, and the balance at least about 70% chromium.

14. A wrought chromium-base alloy characterized by a depressed ductile to brittle transition temperature whereby it has an improved temperature range of malleability, said alloy containing an amount of at least one metal selected from the group consisting of Ru, Pd, Ir and Pt ranging from over about 3 to about 30 atomic percent, less than 0.005% by weight of nitrogen, and the balance at least about 70% chromium.

15. A wrought chromium-base alloy characterized by a depressed ductile to brittle transition temperature whereby it has an improved temperature range of malleability, said alloy containing an amount of at least one metal selected from the group consisting of Ru, Pd, Ir and Pt ranging from about 4 to 20 atomic percent, less than 0.005 by weight of nitrogen, and the balance at least about 70% chromium.

16. A wrought chromium-base alloy characterized by a depressed ductile to brittle transition temperature whereby it has an improved temperature range of malleability,

said alloy containing an amount of ruthenium ranging from about 0.5 to 30 atomic percent at least sutficient to depress the ductile to brittle transition temperature of said alloy less than 0.01% by weight of nitrogen, substantially the balance of said alloy being at least about chromium.

17. A wrought chromium-base alloy characterized by a depressed ductile to brittle transition temperature whereby it has an improved temperature range of malleability, said alloy containing an amount of ruthenium ranging from about 1 to 10 atomic percent, less than 0.005% by weight of nitrogen, the balance of said alloy being at least about 70% chromium.

18. A wrought chromium-base alloy characterized by a depressed ductile to brittle transition temperature whereby it has an improved temperature range of malleability, said alloy containing an amount of iridium ranging from over about 3 to about 20 atomic percent, less than 0.01% by weight of nitrogen, the balance of said alloy being at least about 70% chromium.

19. A wrought chromium-base alloy characterized by a depressed ductile to brittle transition temperature where by it has an improved temperature range of malleability, said alloy containing an amount of iridium ranging from about 7 to 20 atomic percent, less than 0.005% by weight of nitrogen, the balance of said alloy being at least about 70% chromium.

20. A wrought chromium-base alloy characterized by a depressed ductile to brittle transition temperature whereby it has an improved temperature range of malleability, said alloy containing an amount of palladium ranging from over about 3 to about 20 atomic percent, less than 0.01% by weight of nitrogen, the balance of said alloy being at least about 70% chromium.

21. A wrought chromium-base alloy characterized by a depressed ductile to brittle transition temperature whereby it has an improved temperature range of malleability, said alloy containing an amount of palladium ranging from about 7 to 20 atomic percent, less than 0.005% by weight of nitrogen, the balance of said alloy being at least about 70% chromium.

References Cited in the file of this patent Zeitschrift fiir Metallkunde, vol. 46, 1955, pages 210 to 215.

Physikalische Zeitschrift, vol. 36, 1935, pages 188 and 189. 

14. A WROUGHT CHROMIUM-BASE ALLOY CHARACTERIZED BY A DEPRESSED DUCTILE TO BRITTLE TRANSITION TEMPERATURE WHEREBY IT HAS AN IMPROVED TEMPERATURE RANGE OF MALLEABILITY, SAID ALLOY CONTAINING AN AMOUNT OF AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF RU, PD, IR AND PT RANGING FROM OVER ABOUT 3 TO ABOUT 30 ATOMIC PER- 